The Science and Engineering of Materials, 4th ed Donald R. Askeland

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The Science and Engineering of Materials, 4th
edDonald R. Askeland Pradeep P. Phulé

• Chapter 22 Corrosion and Wear

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Objectives of Chapter 22

• To introduce the principles and mechanisms by
  which corrosion and wear occur under different
  conditions. This includes the aqueous corrosion
  of metals, the oxidation of metals, the corrosion
  of ceramics, and the degradation of polymers.
• To give summary of different technologies that
  are used to prevent or minimize corrosion and
  associated problems.

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Chapter Outline

• 22.1 Chemical Corrosion
• 22.2 Electrochemical Corrosion
• 22.3 The Electrode Potential in
  Electrochemical Cells
• 22.4 The Corrosion Current and Polarization
• 22.5 Types of Electrochemical Corrosion
• 22.6 Protection Against Electrochemical
  Corrosion
• 22.7 Microbial Degradation and
  Biodegradable Polymers
• 22.8 Oxidation and Other Gas Reactions
• 22.9 Wear and Erosion

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Section 22.1
Chemical Corrosion

• Chemical corrosion - Removal of atoms from a
  material by virtue of the solubility or chemical
  reaction between the material and the surrounding
  liquid.
• Dezincification - A special chemical corrosion
  process by which both zinc and copper atoms are
  removed from brass, but the copper is replated
  back onto the metal.
• Graphitic corrosion - A special chemical
  corrosion process by which iron is leached from
  cast iron, leaving behind a weak, spongy mass of
  graphite.

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Figure 22.1 Molten lead is held in thick steel
pots during refining. In this case, the molten
lead has attacked a weld in a steel plate and
cracks have developed. Eventually, the cracks
propagate through the steel, and molten lead
leaks from the pot.
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Figure 22.2 Photomicrograph of a copper deposit
in brass, showing the effect of dezincification
(x50).
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Section 22.2
Electrochemical Corrosion

• Electrochemical corrosion - Corrosion produced by
  the development of a current in an
  electrochemical cell that removes ions from the
  material.
• Electrochemical cell - A cell in which electrons
  and ions can flow by separate paths between two
  materials, producing a current which, in turn,
  leads to corrosion or plating.
• Oxidation reaction - The anode reaction by which
  electrons are given up to the electrochemical
  cell.
• Reduction reaction - The cathode reaction by
  which electrons are accepted from the
  electrochemical cell.

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Figure 22.3 The components in an electrochemical
cell (a) a simple electrochemical cell and (b) a
corrosion cell between a steel water pipe and a
copper fitting.
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Figure 22.4 The anode and cathode reactions in
typical electrolytic corrosion cells (a) the
hydrogen electrode, (b) the oxygen electrode, and
(c) the water electrode.
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Section 22.3
The Electrode Potential in Electrochemical Cells

• Electrode potential - Related to the tendency of
  a material to corrode. The potential is the
  voltage produced between the material and a
  standard electrode.
• emf series - The arrangement of elements
  according to their electrode potential, or their
  tendency to corrode.
• Nernst equation - The relationship that describes
  the effect of electrolyte concentration on the
  electrode potential in an electrochemical cell.
• Faradays equation - The relationship that
  describes the rate at which corrosion or plating
  occurs in an electrochemical cell.

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Figure 22.5 The half-cell used to measured the
electrode potential of copper under standard
conditions. The electrode potential of copper is
the potential difference between it and the
standard hydrogen electrode in an open circuit.
Since E0 is great than zero, copper is cathodic
compared with the hydrogen electrode.
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Example 22.1
Half-Cell Potential for Copper
Suppose 1 g of copper as Cu2 is dissolved in
1000 g of water to produce an electrolyte.
Calculate the electrode potential of the copper
half-cell in this electrolyte. Example 22.1
SOLUTION From chemistry, we know that a standard
1-M solution of Cu2 is obtained when we add 1
mol of Cu2 (an amount equal to the atomic mass
of copper) to 1000 g of water. The atomic mass of
copper is 63.54 g/mol. The concentration of the
solution when only 1 g of copper is added must be
From the Nernst equation, with n 2 and E0
0.34 V
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Example 22.2
Design of a Copper Plating Process
Design a process to electroplate a 0.1-cm-thick
layer of copper onto a 1 cm ? 1 cm cathode
surface. Example 22.2 SOLUTION In order for us to
produce a 0.1-cm-thick layer on a 1 cm2 surface
area, the weight of copper must be
From Faradays equation, where MCu 6354 g/mol
and n 2
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Example 22.2 SOLUTION Therefore, we might use
several different combinations of current and
time to produce the copper plate
Our choice of the exact combination of current
and time might be made on the basis of the rate
of production and quality of the copper plate. A
current of 1 A and a time of 45 minutes are
not uncommon in electroplating operations.
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Example 22.3
Corrosion of Iron
An iron container 10 cm ? 10 cm at its base is
filled to a height of 20 cm with a corrosive
liquid. A current is produced as a result of an
electrolytic cell, and after 4 weeks, the
container has decreased in weight by 70 g.
Calculate (1) the current and (2) the current
density involved in the corrosion of the
iron. Example 22.3 SOLUTION 1. The total exposure
time is
From Faradays equation, using n 2 and M
55.847 g/mol
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Example 22.3 SOLUTION 2. The total surface area
of iron in contact with the corrosive liquid and
the current density are
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Example 22.4
Copper-Zinc Corrosion Cell
Suppose that in a corrosion cell composed of
copper and zinc, the current density at the
copper cathode is 0.05 A/cm2. The area of both
the copper and zinc electrodes is 100 cm2.
Calculate (1) the corrosion current, (2) the
current density at the zinc anode, and (3) the
zinc loss per hour. Example 22.4 SOLUTION 1. The
corrosion current is
2. The current in the cell is the same
everywhere. Thus
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Example 22.4 SOLUTION 3. The atomic mass of zinc
is 65.38 g/mol. From Faradays equation
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Section 22.4
The Corrosion Current and Polarization

• Polarization - Changing the voltage between the
  anode and cathode to reduce the rate of
  corrosion.
• Activation polarization is related to the energy
  required to cause the anode or cathode reaction
• Concentration polarization is related to changes
  in the composition of the electrolyte
• Resistance polarization is related to the
  electrical resistivity of the electrolyte.

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Section 22.5
Types of Electrochemical Corrosion

• Intergranular corrosion - Corrosion at grain
  boundaries because grain boundary segregation or
  precipitation produces local galvanic cells.
• Stress corrosion - Deterioration of a material in
  which an applied stress accelerates the rate of
  corrosion.
• Oxygen starvation - In the concentration cell,
  low-oxygen regions of the electrolyte cause the
  underlying material to behave as the anode and to
  corrode.
• Crevice corrosion - A special concentration cell
  in which corrosion occurs in crevices because of
  the low concentration of oxygen.

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Example 22.5
Corrosion of a Soldered Brass Fitting
A brass fitting used in a marine application is
joined by soldering with lead-tin solder. Will
the brass or the solder corrode? Example 22.5
SOLUTION From the galvanic series, we find that
all of the copper-based alloys are more cathodic
than a 50 Pb-50 Sn solder. Thus, the solder is
the anode and corrodes. In a similar manner, the
corrosion of solder can contaminate water in
freshwater plumbing systems with lead.
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Figure 22.6 Example of microgalvanic cells in
two-phase alloys (a) In steel, ferrite is anodic
to cementite. (b) In austenitic stainless steel,
precipitation of chromium carbide makes the low
Cr austenite in the grain boundaries anodic.
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Figure 22.7 Photomicrograph of intergranular
corrosion in a zinc die casting. Segregation of
impurities to the grain boundaries produces
microgalvanic corrosion cells (x50).
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Figure 22.8 Examples of stress cells. (a) Cold
work required to bend a steel bar introduces high
residual stresses at the bend, which then is
anodic and corrodes. (b) Because grain
boundaries have a high energy, they are anodic
and corrode.
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Example 22.6
Corrosion of Cold-Drawn Steel
A cold-drawn steel wire is formed into a nail by
additional deformation, producing the point at
one end and the head at the other. Where will the
most severe corrosion of the nail occur? Example
22.6 SOLUTION Since the head and point have been
cold-worked an additional amount compared with
the shank of the nail, the head and point serve
as anodes and corrode most rapidly.
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Figure 22.9 Concentration cells (a) Corrosion
occurs beneath a water droplet on a steel plate
due to low oxygen concentration in the water. (b)
Corrosion occurs at the tip of a crevice because
of limited access to oxygen.
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Example 22.7
Corrosion of Crimped Steel
Two pieces of steel are joined mechanically by
crimping the edges. Why would this be a bad idea
if the steel is then exposed to water? If the
water contains salt, would corrosion be
affected? Example 22.7 SOLUTION By crimping the
steel edges, we produce a crevice. The region in
the crevice is exposed to less air and moisture,
so it behaves as the anode in a concentration
cell. The steel in the crevice corrodes. Salt
in the water increases the conductivity of the
water, permitting electrical charge to be
transferred at a more rapid rate. This causes a
higher current density and, thus, faster
corrosion due to less resistance polarization.
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Figure 22.10 (a) Bacterial cells growing in a
colony (x2700). (b) Formation of a tubercule and
a pit under a biological colony.
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Section 22.6
Protection Against Electrochemical Corrosion

• Inhibitors - Additions to the electrolyte that
  preferentially migrate to the anode or cathode,
  cause polarization, and reduce the rate of
  corrosion.
• Sacrificial anode - Cathodic protection by which
  a more anodic material is connected electrically
  to the material to be protected. The anode
  corrodes to protect the desired material.
• Passivation - Producing strong anodic
  polarization by causing a protective coating to
  form on the anode surface and to thereby
  interrupt the electric circuit.

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Example 22.8
Effect of Areas on Corrosion Rate for
Copper-Zinc Couple
Consider a copper-zinc corrosion couple. If the
current density at the copper cathode is 0.05
A/cm2, calculate the weight loss of zinc per hour
if (1) the copper cathode area is 100 cm2 and the
zinc anode area is 1 cm2 and (2) the copper
cathode area is 1 cm2 and the zinc anode area is
100 cm2.
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Figure 22.11 Alternative methods for joining two
pieces of steel (a) Fasteners may produce a
concentration cell, (b) brazing or soldering may
produce a composition cell, and (c) welding with
a filler metal that matches the base metal may
avoid the formation of galvanic cells (for
Example 22.8)
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Example 22.8 SOLUTION 1. For the small zinc anode
area
2. For the large zinc anode area
The rate of corrosion of the zinc is reduced
significantly when the zinc anode is much larger
than the cathode.
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Figure 22.12 Zinc-plated steel and tin-plated
steel are protected differently. Zinc protects
steel even when the coating is scratched, since
zinc is anodic to steel. Tin does not protect
steel when the coating is disrupted, since steel
is anodic with respect to tin.
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Figure 22.13 Cathodic protection of a buried
steel pipeline (a) A sacrificial magnesium anode
assures that the galvanic cell makes the pipeline
the cathode. (b) An impressed voltage between a
scrap iron auxiliary anode and the pipeline
assures that the pipeline is the cathode.
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Figure 22.14 (a) Intergranular corrosion takes
place in austenitic stainless steel. (b) Slow
cooling permits chromium carbides to precipitate
at grain boundaries. (c) A quench anneal to
dissolve the carbides may prevent intergranular
corrosion.
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Example 22.9
Design of a Corrosion Protection System
Steel troughs are located in a field to provide
drinking water for a herd of cattle. The troughs
frequently rust through and must be replaced.
Design a system to prevent or delay this
problem. Example 22.9 SOLUTION We might, for
example, fabricate the trough using stainless
steel or aluminum. Either would provide better
corrosion resistance than the plain carbon steel,
but both are considerably more expensive than the
current material. We might suggest using
cathodic protection a small magnesium anode
could be attached to the inside of the trough.
The anode corrodes sacrificially and prevents
corrosion of the steel.
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Example 22.9 SOLUTION (Continued) Another
approach would be to protect the steel trough
using a suitable coating. Painting the steel
(that is, introducing a protective polymer
coating) and, using a tin-plated steel, provides
protection as long as the coating is not
disrupted. The most likely approach is to use a
galvanized steel, taking advantage of the
protective coating and the sacrificial behavior
of the zinc. Corrosion is very slow due to the
large anode area, even if the coating is
disrupted. Furthermore, the galvanized steel is
relatively inexpensive, readily available, and
does not require frequent inspection.
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Example 22.10
Design of a Stainless-Steel Weldment
A piping system used to transport a corrosive
liquid is fabricated from 304 stainless steel.
Welding of the pipes is required to assemble the
system. Unfortunately, corrosion occurs and the
corrosive liquid leaks from the pipes near the
weld. Identify the problem and design a system to
prevent corrosion in the future.
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Figure 22.15 The peak temperature surrounding a
stainless-steel weld and the sensitized structure
produced when the weld slowly cools (for Example
22.10)
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Example 22.10 SOLUTION A portion of the pipe in
the HAZ heats into the sensitization temperature
range, permitting chromium carbides to
precipitate. If the cooling rate of the weld is
very slow, the fusion zone and other areas of the
heat-affected zone may also be affected.
Sensitization of the weld area, therefore, is the
likely reason for corrosion of the pipe in the
region of the weld. We might use a welding
process that provides very rapid rates of heat
input, causing the weld to heat and cool very
quickly. We might heat treat the assembly after
the weld is made. By performing a quench anneal,
any precipitated carbides are re-dissolved during
the anneal and do not re-form during
quenching. Perhaps our best design is to use a
stainless steel that is not subject to
sensitization.
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Section 22.7
Microbial Degradation and Biodegradable Polymers

• Simple polymers (such as polyethylene,
  polypropylene, and polystyrene),
  high-molecular-weight polymers, crystalline
  polymers, and thermosets are relatively immune to
  attack.
• However, certain polymersincluding polyesters,
  polyurethanes, cellulosics, and plasticized
  polyvinyl chloride (which contains additives that
  reduce the degree of polymerization)are
  particularly vulnerable to microbial degradation.

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Section 22.8
Oxidation and Other Gas Reactions

• Oxidation - Reaction of a metal with oxygen to
  produce a metallic oxide. This normally occurs
  most rapidly at high temperatures.
• Pilling-Bedworth ratio - Describes the type of
  oxide film that forms on a metal surface during
  oxidation.

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Figure 22.16 The standard free energy of
formation of selected oxides as a function of
temperature. A large negative free energy
indicates a more stable oxide.
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Example 22.11
Chromium-Based Steel Alloys
Explain why we should not add alloying elements
such as chromium to pig iron before the pig iron
is converted to steel in a basic oxygen furnace
at 1700oC. Example 22.11 SOLUTION In a basic
oxygen furnace, we lower the carbon content of
the metal from about 4 to much less than 1 by
blowing pure oxygen through the molten metal. If
chromium were already present before the steel
making began, chromium would oxidize before the
carbon (Figure 22.16), since chromium oxide has a
lower free energy of formation (or is more
stable) than carbon dioxide (CO2). Thus, any
expensive chromium added would be lost before the
carbon was removed from the pig iron.
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Figure 22.17 Three types of oxides may form,
depending on the volume ratio between the metal
and the oxide (a) magnesium produces a porous
oxide film, 9b) aluminum forms a protective,
adherent, nonporous oxide film, and (c) iron
forms an oxide film that spills off the surface
and provides poor protection.
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Example 22.12
Pilling-Bedworth Ratio
The density of aluminum is 2.7 g/cm3 and that of
Al2O3 is about 4 g/cm3. Describe the
characteristics of the aluminum-oxide film.
Compare with the oxide film that forms on
tungsten. The density of tungsten is 19.254 g/cm3
and that of WO3 is 7.3 g/cm3. Example 22.12
SOLUTION For 2Al 3/2O2 ? Al2O3, the molecular
weight of Al2O3 is 101.96 and that of aluminum is
26.981.
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Example 22.12 SOLUTION For tungsten, W 3/2O2 ?
WO3. The molecular weight of WO3 is 231.85 and
that of tungsten is 183.85
Since P-B 1 for aluminum, the Al2O3 film is
nonporous and adherent, providing protection to
the underlying aluminum. However, P-B gt 2 for
tungsten, so the WO3 should be nonadherent and
nonprotective.
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Example 22.13
Parabolic Oxidation Curve for Nickel
At 1000oC, pure nickel follows a parabolic
oxidation curve given by the constant k 3.9 ?
10-12 cm2/s in an oxygen atmosphere. If this
relationship is not affected by the thickness of
the oxide .lm, calculate the time required for a
0.1-cm nickel sheet to oxidize completely. Example
22.13 SOLUTION Assuming that the sheet oxidizes
from both sides
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Section 22.9
Wear and Erosion

• Adhesive wear - Removal of material from surfaces
  of moving equipment by momentary local bonding,
  then bond fracture, at the surfaces.
• Abrasive wear - Removal of material from surfaces
  by the cutting action of particles.
• Cavitation - Erosion of a material surface by the
  pressures produced when a gas bubble collapses
  within a moving liquid.
• Liquid impingement - Erosion of a material caused
  by the impact of liquid droplets carried by a gas
  stream.

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Figure 22.18 The asperities on two rough
surfaces may initially be bonded. A sufficient
force breaks the bonds and the surfaces slide.
As they slide, asperities may be fractured,
wearing away the surfaces and producing debris.
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Figure 22.19 Abrasive wear, caused by either
trapped or free-flying abrasives, produces
troughs in the material, piling up asperities
that may fracture into debris.
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Figure 22.20 Two steel sheets joined by an
aluminum rivet (for Problem 22.25).
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Figure 22.21 Cross-section through an integrated
circuit showing the external lead connection to
the chip (for Problem 22.26).